U.S. Pat. No. 4,102,664 provides the basic disclosure of a method for making glass articles exhibiting defect-free surfaces which can be equivalent in smoothness to that produced through a polishing operation. As is explained therein, the method comprises five general steps:
(1) two glass forming batches of different compositions are melted, one batch for a glass which is highly soluble in a given solvent and the second batch for a glass which is relatively insoluble in the same solvent;
(2) these molten batches are simultaneously brought together while in the fluid state, i.e., at viscosities no higher than 2.5.times.10.sup.5 poises, to form a laminated glass body wherein the insoluble glass is covered by the soluble glass;
(3) the glass layers are fused together at a temperature where they are sufficiently fluid to provide an interface therebetween which is defect free;
(4) the laminated article is cooled; and thereafter
(5) the soluble glass layer is dissolved away in an appropriate solvent.
As is apparent from the above description, the basic concept of the process involved forming a glass body of a desired geometry having a main or core glass portion substantially enveloped within a skin of a second glass, this second glass having a composition exhibiting high solubility in a particular liquid. Hence, the core body is not exposed to any environmental abuse during the initial forming operation, during any subsequent forming operation, or during the cooling operation. Accordingly, it is protected from the occurrence of mechanical and/or chemical surface flaws inherently produced in glass forming processes. Consequently, an essentially pristine, defectfree surface on the core glass portion results when the skin glass is dissolved away.
The most basic requirement underlying the efficacy of the patented method is the presence of two glass compositions exhibiting very different solubilities in a particular solvent. As observed there, the skin glass will be at least ten times more soluble than the core glass, and preferably greater than 100 times more soluble. Whereas the disclosure of the patent posited the operability of forming a body glass demonstrating high resistance to attack by bases and a skin glass highly soluble in alkaline solutions, and the operability of forming a core glass exhibiting high resistance to attack by hydrofluoric acid and a skin glass highly soluble in hydrofluoric acid, the most practical combination of glasses was deemed to contemplate a body glass designed for resistance to water and/or dilute acids and a skin glass highly soluble therein.
The patent disclosed three glasses illustrative of suitable body glasses: the first had a composition generally within the alkali metal, alkaline earth metal (predominantly Sr) silicate system; the second was composed generally of alkali metal aluminosilicate; and the third consisted principally of alkali metal zinc silicate. As illustrative of operable skin glasses, the patent referred to glasses having compositions in the following systems: alkali metal silicates; various metaphosphate-type compositions; barium borosilicates; and various borate-based compositions. In a preferred embodiment the body and skin glass compositions were so selected that an exchange of ions occurred between the two when brought into contact with each other and/or when the laminated body was subsequently heat treated, that exchange enhancing the solubility of the skin glass and/or increasing the mechanical strength of the core glass. As illustrations of that phenomenon the patent noted the possible exchange of Ca.sup.+2 ions from the skin glass with Na.sup.+ ions from the body glass, the presence of the Na.sup.+ ions rendering the skin glass more soluble, and the possible exchange of Li.sup.+ ions from the skin glass with Na.sup.+ and/or K.sup.+ ions from the body glass, the presence of the Li.sup.+ ions leading to the formation of a surface layer on the body glass having a lower coefficient of thermal expansion than the original body glass composition.
With respect to processing parameters for forming the laminated precursor glass article, the disclosure of the patent specified the following three requirements:
(a) the coefficient of thermal expansion of the skin glass from room temperature to the set point thereof should not exceed that of the core glass; preferably it will be somewhat lower;
(b) the viscosity of the skin glass will be equal to or somewhat lower than that of the body glass; and
(c) the liquidus temperatures of both the skin and core glasses will most preferably be below the lamination temperature in order to avoid the occurrence of devitrification.
The patent expressly illustrates one forming process, viz., downdrawing in pairs two streams of glasses of different compositions to form cane having a core portion enveloped within a skin. However, the patent notes that shaping of laminated articles can be performed in any conventional glass forming practice, specifically mentioning pressing, vacuum forming, and press-and-blow.
As is well recognized in the art, soda lime glass is utilized in the fabrication of containers and window glass and, as such, comprises by far the greatest tonnage of glass melted. The compositions of commercial soda lime glasses vary somewhat over relatively narrow ranges of individual components, such variations frequently depending upon the batch materials employed. A typical analysis of sheet glass manufactured through the float glass process is reported below in weight percent, along with several physical properties measured thereon and the predominant crystal phase developed therein upon devitrification thereof.
______________________________________ SiO.sub.2 73.33 MgO 3.91 Na.sub.2 O 13.69 Al.sub.2 O.sub.3 0.11 K.sub.2 O 0.03 SO.sub.3 0.15 CaO 8.79 Fe.sub.2 O.sub.3 0.087 Softening Point (10.sup.7.6 poises) 727.degree. C. Annealing Point (.apprxeq.10.sup.13 poises) 543.degree. C. Strain Point (.apprxeq.10.sup.14.5 poises) 504.degree. C. Density 2.494 g/cm.sup.3 Coefficient of Thermal Expansion 86.2 .times. 10.sup.-7 /.degree.C. (25.degree.-300.degree. C.) Coefficient of Thermal Expansion 89.6 .times. 10.sup.-7 /.degree.C. (25.degree.-450.degree. C.) Liquidus Temperature 992.degree. C. Crystal Phase Wollastonite ______________________________________
None of the composition systems of skin or cladding glasses disclosed in U.S. Pat. No. 4,102,664 appeared to be useful for combination with soda lime glasses to develop defect-free surfaces thereon. Accordingly, the primary objective of the present invention was to devise glass compositions suitable for cladding articles prepared from soda lime glass to protect the surface thereof as it is being formed into a desired shape, but which are more readily soluble in a particular solvent than soda lime glass, such as to be easily removable from the soda lime body shape. A second objective was to devise an improved method for forming laminated articles consisting of soda lime body glass and a soluble skin glass.